1. Field of the Invention
The present invention relates to a process for producing molded articles of a polyimide precursor from which there can be obtained molded articles of polyimide having excellent heat-resistance by carrying out a heat treatment.
More specifically, the present invention relates to a process for producing molded articles of a polyimide precursor which is scarcely imidated and which has self-shape retention properties for inhibiting fluidization at room temperature.
2. Description of the Related Art
Polyimides have excellent heat resistance, chemical resistance, mechanical properties and electrical properties. Polyimide films can usually be produced by applying a solution containing 8 to 30% of a polyamic acid (a polyimide precursor or PAA) in a fluid state to a support by casting or by the use of a T-die; drying the solution by hot air or the like to obtain fluidity-free PAA films having self-shape retention properties; and then further heating the films up to about 400.degree. C. to produce the polyimide films. For example, as the polyimide films, there is a trade name "Capton" made by Du Pont.
However, since these polyimide films are manufactured by a solution casting process, productivity is poor and cost is high. On the other hand, in order to improve the productivity, there is a process which comprises completely removing a solvent; melting and extruding a pelletized thermoplastic polyimide at a high temperature (e.g., 400.degree. C.); and then cooling the extruded articles to produce the films (e.g., Japanese Patent Application Laid-open No.Hei-3-42224).
The thus produced thermoplastic polyimide films, since being thermoplastic, has poorer heat resistance as compared with a thermosetting polyimide obtained by curing a polyimide precursor.
Japanese Patent Application Laid-open No.Sho-63-243321 has disclosed a method which comprises the steps of obtaining a polyimide precursor as a polyamic acid ester by solution polymerization, concentrating or isolating it under reduced pressure, adjusting the concentration of the ester to 30-80% by weight, heating/dissolving, extruding, cooling it to obtain molded articles, and then heating them at a high temperature (300.degree. C. or more) to obtain polyimide molded articles.
However, this method requires the concentration of the precursor, and so the reaction of esterifying an aromatic tetracarboxylic acid is necessary. In consequence, the process is complex. Additionally, in this method, N-methylpyrrolidone having a high viscosity and a high boiling point (204.degree. C.) is used as a solvent, and therefore a long period of time is required to merely concentrate PAA under reduced pressure, so that yield is poor.